The present invention relates to a nickel powder for multilayer ceramic capacitors suitable as a material of conductive paste fillers for forming internal electrodes in multilayer ceramic capacitors.
Conductive metal powders such as nickel, copper, silver, etc., are useful for forming internal electrodes in multilayer ceramic capacitors, and in particular, nickel powder has attracted attention since it is less expensive than conventional metal powders such as palladium powder, etc. Additionally, internal electrodes tend to be formed in thin layers having a thickness of 1 to 2 xcexcm in accordance with miniaturization and capacity increase of capacitors, and therefore, it is required that nickel powder have a particle size of not more than 1 xcexcm.
As nickel powder which satisfies such properties, fine spherical nickel powder having a purity of 99.5% or more by weight and a particle size of 0.05 to 1.0 xcexcm is disclosed in Japanese Unexamined Patent Publication No. Hei 3-280304. According to this nickel powder, since filling density in the electrode layer can be increased, specific resistance of the electrode layer after sintering is small and delamination (peeling) and cracking hardly occur.
In addition, as a process for production of nickel powder, a vapor phase reduction in which nickel chloride gas is reduced by hydrogen gas is generally used, recently. This method has an advantage in that spherical nickel powders can be efficiently produced; however, nickel powders having a particle size of 1 xcexcm or more are often included in the produced nickel powders. The nickel powder having a particle size of 1 xcexcm or more easily causes problems such as shorting between electrodes or voids in the electrode layer, and it does not have desirable properties. Therefore, high-quality nickel powder having no coarse particles of 1 xcexcm or more is desired. Since nickel powder particles of 1 to 2 xcexcm or 5 xcexcm, depending on the conditions, are included even if an average particle size thereof is 0.4 xcexcm, a classification technique in which coarse particles of 1 xcexcm or more can be efficiently removed is desired.
The present invention has been made in consideration of the above circumstances, and objects thereof are as follows.
{circle around (1)} A nickel powder for multilayer ceramic capacitors is provided in which the content of coarse particles is low and the distribution of particle size is narrow.
{circle around (2)} A nickel powder for multilayer ceramic capacitors is provided in which the surface roughness in the paste state is small.
The present inventors reduced nickel chloride by a vapor phase reduction, then prepared nickel powders in the paste state with respect to each nickel powder in which the contents of nickel powders having a particle size of 2 xcexcm or more differ, and examined the surface roughness thereof. As a result, it was found that satisfactory surface roughness was not obtained in the case in which nickel powders having a particle size of 2 xcexcm or more exist at a rate of about 1400 per million; however, superior surface roughness was obtained in the case in which the nickel powders exist at a rate of about 50 per million. It was inferred from this fact that the surface roughness in which there is no problem in practical use would be obtained if nickel powder were classified until nickel powders having a particle size of 2 xcexcm or more exist at a rate of 700 per million, which is about the intermediate value between 50 per million and 1400 per million, and the present invention therefore was attained. That is, a nickel powder for multilayer ceramic capacitors according to the present invention is characterized in that an average particle size is 0.1 to 1.0 xcexcm, and the nickel powders having a particle size of 2 xcexcm or more exist at a rate of not more than 700 per million.
Next, the present invention is more specifically explained.
A. Nickel Powder Characteristics
The nickel powder of the present invention is characterized in that an average particle size is 0.1 to 1.0 xcexcm, and the number rate of the nickel powders having a particle size of 2 xcexcm or more is not more than 700 per million, as described above. When such nickel powder is used for the paste, the uniformity of thickness of the internal electrode layer in ceramic capacitors is superior, and shorting between internal electrodes is thereby improved. The content (number rate) of nickel powder having a particle size of 2 xcexcm or more can be obtained by taking and image-analyzing electron microphotographs of nickel powder, and counting the total number of particles and particles having a particle size of 2 xcexcm or more.
The number rate of the nickel powders having a particle size of 2 xcexcm or more must be not more than 700 per million, and it is preferable that the number rate is not more than 300 per million, more preferably that the number rate is not more than 100 per million, and most preferably that the number rate is not more than 50 per million.
It is preferable that the average particle size of the nickel powder be even smaller; however, the nickel powders easily aggregate as they are made finer, and consequently, voids are easily formed in internal electrodes. Therefore, as nickel powder for forming an internal electrode of about 1 xcexcm, a particle having an average particle size of 0.2 to 0.4 xcexcm is desirable.
With respect to this nickel powder particle having an average particle size of 0.2 to 0.4 xcexcm, the content (number rate) of the nickel powders having a particle size of 2 xcexcm or more is preferably not more than 50 per million, and more preferably that the number rate is not more than 20 per million. Furthermore, the number rate of the nickel powders having a particle size of 1 xcexcm or more is preferably not more than 100 per million, and more preferably that the number rate is not more than 50 per million.
B. Preparation of Nickel Powder
The above nickel powder of the present invention can be prepared by various methods, and in particular, the vapor phase reduction in which nickel chloride is reduced by hydrogen, etc., is a preferable method from the viewpoint of control of particle size. Specifically, the following processes may be mentioned.
{circle around (1)} Nickel chloride solid as a starting material is vaporized by heating and reacts with hydrogen gas (reduction process), and nickel powder is therefore obtained.
{circle around (2)} Nickel solid as a starting material is chlorinated by contacting chlorine gas (chlorination process), nickel chloride gas is thereby produced and reacts with hydrogen gas (reduction process), and nickel powder is therefore obtained.
In the process {circle around (2)} of the above processes, the partial pressure of nickel chloride gas can be controlled in the reduction process by controlling the feed rate of chlorine gas in the chlorination process or by mixing nickel chloride gas produced in the chlorination process with inert gas. Thus, by controlling the partial pressure of nickel chloride gas, the particle size of forming nickel powder can be controlled, and as a result, the particle size of nickel powder can be stabilized and can be optionally controlled.
Incidentally, the reduction process in the above vapor phase reduction is carried out at a high temperature of about 1000xc2x0 C. or more. The nickel powder just after production is easily aggregated since it has a high temperature, and it is therefore desirable that it cool rapidly. Specifically, a process in which the produced nickel powder is forced to cool by inert gas such as nitrogen gas, etc., may be mentioned. As a cooling process, a cooling device, etc., can be provided and be used in addition to the reduction reaction system, and in particular, it is desirable that inert gas for cooling be directly contacted with nickel powder just after production from the viewpoint of suppression of the aggregation of nickel powder. In such a cooling process, the cooling rate in which the nickel powder just after production is forced to cool is preferably 30xc2x0 C./second or more, more preferably 40xc2x0 C./second or more, and most preferably 50 to 200xc2x0 C./second, and in addition, the temperature to which it is to be cooled, from a temperature level of the reduction reaction, is preferably at least not more than 800xc2x0 C., more preferably not more than 600xc2x0 C., and most preferably not more than 400xc2x0 C. In addition, after such cooling, it is also preferable to further cool the nickel powder to a temperature which is lower than the above temperature (for example, a temperature ranging from room temperature to about 150xc2x0 C.) at the same cooling rate.
Thus, a nickel powder of the present invention in which few coarse particles having a particle size of 2 xcexcm or more are included can be obtained by controlling reaction conditions and cooling conditions for producing nickel powder.
In order to obtain the nickel powder of the present invention, it is preferable that the above produced nickel powder be further classified by mechanical means and that the coarse particles be removed. In the following, the specific classification process is explained.
C. Classification Process for Nickel Powder
As a process for classifying nickel powder having a particle size of 2 xcexcm or more, that is, coarse particles, general classifiers such as hydrocyclones, air classifiers, etc., may be employed. However, since hydrogen chloride gas or nickel chloride is adhered or adsorbed at the surface of the produced nickel powder and must usually be removed by washing, a process in which the nickel powder is dispersed in water and is prepared in the slurry form and is then classified by removing coarse particles using a hydrocyclone, is desirable.
D. Hydrocyclone
As a hydrocyclone, a two-liquid separating type in which fine particles are discharged from the top portion of the apparatus and coarse particles are discharged from the bottom portion thereof, and a three-liquid separating type in which super-fine particles, which are finer than fine particles, are discharged from the top portion of the apparatus, fine particles are discharged from the middle portion thereof, and coarse particles are discharged from the bottom portion thereof, can be mentioned. Of these, the latter type is more preferably used from the viewpoint of the particle size being delicately controlled. The nickel powder discharged from the top portion and/or the middle portion is defined as a nickel powder of the present invention.
It is preferable that material for the hydrocyclone be ceramic in order to ensure corrosion resistance and wear resistance. As a ceramic, alumina or silicon nitride is desirable. The hydrocyclone may be operated alone or in combination which two or more hydrocyclones in parallel, and mass production can be realized by parallel operation of two or more hydrocyclones, and the productivity can thereby be effectively improved.
E. Slurry
As a medium when the nickel powder is prepared in the slurry form, water can be preferably used as described above. That is, the slurry is obtained by dispersing nickel powder having an average particle size of 0.1 to 1.0 xcexcm into water and is fed into the hydrocyclone. The content of nickel powder in the slurry (slurry concentration) is preferably 5 to 25% by weight, more preferably 7 to 20% by weight, and most preferably 8 to 15% by weight.
In the case in which the above three-liquid separating type of hydrocyclone is used, it is not desirable that the slurry concentration of nickel powder be less than 5% by weight since coarse particles would be easily included in the slurry discharged from the top portion and/or the middle portion. Furthermore, as the slurry concentration is increased, the content of coarse particles in the slurry discharged from the top portion and the middle portion is rapidly decreased; however, when it is more than 20% by weight, the classification efficiency decreases and many coarse particles are included in nickel powder. When the content of the nickel powder in the slurry is within the above range, in particular, in the case in which it is 7 to 20% by weight and is preferably 8 to 15% by weight, it is desirable because the manufacturing efficiency and the classification efficiency are remarkably superior. By classifying nickel powder under such conditions, a nickel powder in which the number rate of coarse particles having a particle size of 2 xcexcm or more is not more than 50 particles per million particles and is extremely low, can be obtained.
F. Discharge Rate of Nickel Powder from Hydrocyclone
With respect to the discharge rate of nickel powder from the above hydrocyclone, a discharge rate in which 5 to 80% of nickel powder in slurry fed into the hydrocyclone is discharged from a nozzle for discharging fine particles of the hydrocyclone and the remainder is discharged from a nozzle for discharging coarse particles, is preferable. In this case, the hydrocyclone is a two-liquid separating type, and the nozzle for discharging fine particles means the above top portion and the nozzle for discharging coarse particles means the bottom portion.
As a more preferable discharge rate, the nickel powder discharged from the top portion (nozzle for discharging super fine particles) and/or the middle portion (nozzle for discharging fine particles) of the hydrocyclone, using the above three-liquid separating type of hydrocyclone, is preferably 5 to 80% by weight and more preferably 5 to 70% by weight. In this case, it is the most desirable that the nickel powder further discharged from the middle portion be 20 to 75% by weight. In the case in which coarse particles must be classified more precisely, it is desirable that the nickel powder discharged from the middle portion be prepared in the slurry form according to the present invention and be fed into the hydrocyclone.
Thus, the nickel powder in the slurry discharged from the top portion and/or the middle portion of the hydrocyclone is separated from water by decantation, filtration, etc., then some processes such as drying, etc., are carried out, and a nickel powder of the present invention is therefore obtained.
The nickel powder is added to organic solvent such as terpineol, decanol etc., and cellulose-type of organic resin such as ethyl cellulose, etc., and is mixed, plasticizers such as phthalic ester etc., are further added therein, and a conductive paste is thereby prepared, and in addition, an internal electrode in a multilayer ceramic capacitor is formed by the conductive paste. A nickel powder of the present invention can prevent failures such as shorting, delamination, etc., due to the roughness of the surface of the electrode, when it is used as an internal electrode in a multilayer ceramic capacitor, because coarse particles having a particle size of 2 xcexcm or more are very rare.